Process for the remediation of soil polluted by organic compounds

ABSTRACT

An improved process is described for the removal of organic pollutants from soil by extraction with a lipophylic solvent, wherein the improvement consists in the fact that the purified soil is treated with water to remove the residual solvent. This process allows the residual solvent to be completely removed from the purified soil enabling it to be re-inserted into the environment, with a low energy consumption and a greater guarante of safety in the running of the plant.

The present invention relates to a process for the remediation of soilpolluted by organic compounds.

In particular, the present invention relates to an improved process forthe removal of organic pollutants from soil by extraction with alipophylic solvent, wherein the improvement consists in the fact thatthe purified soil is treated with water to remove the residual solvent.

The problem of soil contaminated by organic pollutants such ashydrocarbons, polynuclear aromatics, organo-chlorinated products isbecoming increasingly more dramatic in industrialized countries, notonly in terms of interventions on pollution of soil and undergroundwater, but also due to the necessity of recovering areas for possibleindustrial and civil use. Another problem, which is typical of the oilindustry, is the treatment of drill cuttings before being dischargedinto the sea.

There are many sources of pollution and various characteristics of theground subject to pollution. These circumstances make it necessary toavail of a wide range of possible solutions.

The technologies generally adopted for the treatment of soil are: soilwashing, bioremediation and solidification/stabilization.

The term soil-washing refers to washing techniques which use water,optionally containing surfactants.

Soil washing exploits the tendency of pollutants to bind themselves,either chemically or physically, to the finer particles of the sediment.These particles can be separated from the rest of the matrix by means ofa more or less complex series of separation operations based ondifferences in dimensions, density and surface properties. With thistechnology the separation is obtained of a fine polluted fraction and acoarse decontaminated fraction.

The finer particles thus separated, only represent a small fraction ofthe initial polluted volume, but contain most of the pollutants.

Soil washing is an emerging technology adopted in numerous situationsfor reducing the quantity of dangerous material to be deposited in dumpsand recovering part of the treatment costs by the selling of the coarserfraction produced as building material.

This process however has substantial disadvantages deriving from thenecessity of operating with large volumes of extracting agent and thedifficulty of recovering the additives used.

Biological remediation techniques are limited to biodegradable compoundsand non-toxic concentrations and are greatly jeopardized by the longtimes necessary for completing the remediation.

Solidification/stabilization processes consist in adding a hydraulicligand to the sediment, for example Portland cement. In this way thepollutants are physically blocked inside the cementitious matrix and canno longer migrate into the surrounding environment.

In some processes, in addition to cement, specific additives are added,capable of reacting with the pollutants transforming them into lesstoxic or less mobile species.

This type of technology has the disadvantage of producing an increase inthe volume of the material treated (sediment+ligand) and, in any case,does not guarantee the stability of the end-product over the time.

Treatment processes based on extraction with a solvent selected, forexample, from methylene chloride, liquefied C₂-C₄ hydrocarbons, orsupercritical fluids, are also known in the art.

The applicability and efficiency of these processes however is greatlylimited by the presence of high percentages of fine particles and by thehigh water content which generally characterize some types of soil.

In order to overcome these disadvantages, processes have been proposedin the art, for the remediation of soil having a high water content,which are based on the use of an extracting mixture consisting of asolvent-cosolvent couple in such proportions as to form a single phasewith the water contained in the ground (U.S. Pat. No. 5,585,002) or asingle solvent, preferably ethyl acetate (IT 1,298,543).

In particular, according to these processes, the soil, after extractionwith the organic solvent and decanting, are subjected to drying toremove the residual solvent which is present in a quantity generallyranging from 3 to 10%.

For this purpose, equipment is used, which, for reasons of safetyassociated with the flammability of the solvent and to reduce emissionsinto the environment, must be inertized with nitrogen and provided withsuitable powder-capturing systems. Furthermore, due to the low exchangecoefficient of the ground, this equipment requires a considerablethermal exchange surface, a high duty and high temperatures which cancause alterations in the pedologic characteristics of the ground,jeopardizing the possibility of re-insertion in the original site.

It has now been found that by treating the soil, after extraction with asolvent and decanting, with a sufficient quantity of water fordissolving the residual solvent, it is possible to separate from thedispersion thus formed, a surnatant aqueous phase, containing thesolvent, and a solid sedimented phase in which the residual solvent isreduced to a minimum concentration which is such as to allowre-insertion in the environment.

The solvent contained in the above aqueous phase can be recovered bysimple evaporation and with a lower energy consumption, as it is nolonger necessary to heat the whole mass of soil.

In accordance with this, an objective of the present invention relatesto a process for the removal of organic pollutants from soil containingwater in a quantity ranging from 10% to the saturation limit of thefield, which comprises the following steps:

-   (a) mixing of the soil with a lipophylic solvent;-   (b) removal by decanting or separation of a liquid phase, containing    the pollutants and solvent, from a solid phase consisting of the    treated soil;-   (c) recovery of the solvent from the liquid phase obtained in    step (b) by distillation and recirculation in step (a);-   (d) mixing of the solid phase separated in step (c) with water in a    ratio which is such as to completely dissolve the residual solvent;-   (e) separation by decanting of a solid fraction, consisting of the    washed soil, and an aqueous phase saturated with solvent;-   (f) dewatering of the solid fraction obtained in step (e); and-   (g) recovery of the solvent from the aqueous phase by evaporation    and recycling to step (a).

Various kinds of soil can be used in the process and with differentpedologic characteristics, with both a high sandy and slime-claycomponent. It is important to control that the water content present inthe ground to be recovered is close to its field capacity. In the caseof ground with a lower humidity content, the material should be dampenedbefore treatment.

With the process described, clay-type or medium mixture soil istypically treated, with a content of 10 to 50% (with respect to the dryweight) of particles having a particle-size of less than 63 μm and awater content ranging from 10 to 60% by weight.

In step (a), the weight ratio solvent/soil ranges from 0.2 to 5.0,preferably from 0.5 to 1. In a process operating in continuous, thecountercurrent flow-rate ratio solvent/soil ranges from 0.2 to 0.5.

This step is generally carried out at room temperature, for a timeranging from 10 to 120 minutes.

The solvents used in the process according to the present invention areselected so as to be non-toxic and volatile, inexpensive and easilyavailable.

Examples of lipophylic solvents suitable for the purposes of the presentinvention are ethyl acetate and petroleum ether. Among lipophylicsolvents, ethyl acetate is preferred for the purposes of the invention.

Steps (a) to (e) of the process according to the present invention canbe repeated several times, if necessary.

The extraction with a solvent according to step (a) can be carried outwith a batch or continuous process. In the former case, the equipmentconsists of an industrial mixing system such as a plough, blade orrotating drum mixer, into which the quantity of earth to be treated ischarged, together with the solvent, mixing for the period of timenecessary for obtaining the extraction of the pollutants.

In the case of a continuous running of the process, the extractionequipment can consist of a screw system in which the solvent and earthare conveyed in countercurrent. The last screw separates the earth frommost of the solvent.

The separation according to step (b) takes place instantaneously afterthe mixing has been stopped due to the fact that the fine fraction inthe soil is kept in an agglomerated form from the water and only aminimum part passes into the extraction solvent.

In this way, a soil is obtained, purified from pollutants, consisting ofboth the coarse fraction and fine fraction, containing 3-10% of residualsolvent and a liquid fraction containing the pollutants.

In step (c) of the process according to the present invention, the solidphase is washed with water to allow the recovery of the residualextraction solvent, which has a solubility in water of 8.7% (v/v); thesolvent thus recovered can be recycled to step (a) of the processaccording to the present invention.

In the case of a batch process, the washing with water can be carriedout in the same equipment used for the extraction in step (a), byfeeding water to the system after separating the solvent by decanting.Using a continuous process, the washing with water can be effected in asuitable mixer or in an additional screw in which the earth and waterare fed in countercurrent.

In both cases, it is not necessary to pressurize the apparatus withinert gas and no powders are formed, as the material is dispersed inwater.

In step (e) of the process according to the present invention, afterdecanting a solid fraction is separated, consisting of the washed soil,and an aqueous fraction saturated with solvent. The solid fraction thusobtained is then subjected to dewatering, using equipment known in theart, such as belt-pressing, centrifugation.

The recovery of the exhausted solvent (step g) can be effected using athin film evaporator with scraped walls operating at atmosphericpressure or under slight depression, obtaining a boiler residuecontaining the fraction extracted from the soil and minimum quantitiesof water and a stream at the head consisting of the extraction solventaccording to step (a).

Alternatively, said recovery can be effected by distillation in thepresence of water, obtaining a boiler residue consisting of water andpollutants, which can be separated by decanting, and a stream at thehead consisting of the extraction solvent according to step (a).

The process according to the present invention allows the pollutants tobe removed from soil with high water contents and fine particles in asimple and economic way.

FIG. 1 indicates the block scheme of the process according to thepresent invention.

The following examples are illustrative but do not limit the scope ofthe invention in any way.

EXAMPLE 1

A sample of soil removed from a site polluted by hydrocarbons has thefollowing characteristics:

-   pH in aqueous solution=7.02; in KCl 1N=7.34-   Humidity content: 18.3%-   Conductivity: 1.14 mS/cm-   Saline concentration: 6.76 mEq/100 g-   Total hydrocarbons (Total Petroleum Hydrocarbons—TPH) 18,000 ppm    (gas chromatographic determination)-   Agronomic characterization: Gravel 46.3%, Sand 42.4%, clay 11.3%.

A kilogram of this earth are placed in a rotating cylinder together with1 kg of ethyl acetate and maintained under stirring, at roomtemperature, for 30 minutes.

After the mixture has been left to settle for 5 minutes, the liquidphase is filtered on a Teflon filter with 0.45 micron pores, in order todetermine, by weighing, the fine fraction of the soil entrained by thesolvent, which amounts to 0.7% by weight of earth.

The ethyl acetate is recovered from the liquid fraction by distillationin a rotating evaporator at 90° C.

940 g of solvent are recovered, which, with the addition of a further 60g of fresh ethyl acetate are used for a second washing cycle of theearth.

The solvent recovered after this second cycle amounts to 1 kg.

Water (1 kg) is added, in the same rotating drum, to the earth,containing 60 g of residual ethyl acetate, which is sufficient todissolve the quantity of residual solvent, it being known that thesolubility of ethyl acetate in water is equal to 8.7% (v/v).

After 10 minutes of stirring, the mixture is left to settle for 30minutes. The upper aqueous layer is separated and preserved. Thedispersion of earth in water remaining on the bottom is centrifuged at3,000×G, recovering a solid fraction consisting of purified earth inwhich the concentration of Total Petroleum Hydrocarbons (TPH) amounts to180 ppm, within the limits established by the Laws in force.

The aqueous phase obtained from the centrifugation, joined to that putaside after the sedimentation, is heated to 90° C. in a rotatingevaporator, recovering the ethyl acetate dissolved.

EXAMPLE 2 (COMPARATIVE)

A 1 kg sample of soil having the characteristics indicated in Example 1,is dried in the air for 48 hours to reduce the humidity content to 3.2%and then treated as described in Example 1.

The entrainment into the solvent of 65 g of the fine fraction of thesample, is observed. This dispersion remains stable for various hours,making it necessary to effect centrifugation to separate the solid fromthe liquid.

The solvent recovered by distillation is used for a second washing ofthe soil. After the second centrifugation, the two fine fractions of thesoil are joined to the coarse fraction and dried in the air for 24hours.

The concentration of TPH after the second treatment amounts to 1670 ppm,which is a value higher than the maximum limit of 750 ppm indicated bythe Law for re-use of the site (Official Bulletin of the Region ofTuscany nr. 36 of Jun. 16, 1993).

EXAMPLE 3

A sample of soil is dried as in Example 2 and water is then added so asto bring the humidity content to 18%.

After the treatment as described in Example 1, it is observed that thesolvent, obtained by simple decanting, contains less than 0.7% of finefraction.

An analysis of the TPH indicates a residual concentration of 183 ppm, avalue which confirms the results of Example 1, both in terms of groundremediation and simplicity of the recovery operations of the solvent.

The recovery of the solvent, on summing the quantity obtained bydecanting after the extraction treatment of the pollutants andseparation by distillation and by distillation of the residue in thewashing aqueous phase of the soil, proved to be practicallyquantitative.

1. A process for the removal of organic pollutants from soilcharacterized by a water content ranging from 10% to the saturationlimit of the soil, which comprises: (a) mixing of the soil with alipophylic solvent; (b) removal by decanting or separation of a liquidphase, containing the pollutants and solvent, from a solid phaseconsisting of the treated soil; (c) recovery of the solvent from theliquid phase obtained in step (b) by distillation and recirculation instep (a); (d) mixing of the solid phase separated in step (c) with waterin a ratio which is such as to completely dissolve the residual solvent;(e) separation by decanting of a solid fraction, wherein the solidfraction comprises the soil which was washed by the water in (d), and anaqueous phase saturated with solvent; (f) dewatering of the solidfraction obtained in step (e); and (g) recovery of the solvent from theaqueous phase by evaporation and recycling to step (a).
 2. The processaccording to claim 1, wherein the lipophylic solvent is ethyl acetate.3. The process according to claim 1, wherein the weight ratio betweenthe lipophylic solvent and the soil ranges from 0.2 to 5.0.
 4. Theprocess according to claim 3, wherein the weight ratio between thelipophylic solvent and the soil ranges from 0.5 to
 1. 5. The processaccording to claim 1, wherein in step (a) the mixing of the componentsis carried out at room temperature, for a time ranging from 10 to 60minutes.
 6. The process according to claim 1, wherein the separation instep (b) takes place by decanting of the liquid.
 7. The processaccording to claim 1, wherein the recovery of the solvent in step (g) iscarried out with a thin film evaporator with scraping walls operating atatmospheric pressure or under slight depression or by distillation inthe presence of water.
 8. The process according to claim 1, wherein thesolvent recovered in step (d) is recycled to step (a).
 9. The processaccording to claim 1, wherein steps (a) to (e) can be repeated severaltimes.
 10. The process according to claim 1, wherein the organicpollutants arc aliphatic and aromatic hydrocarbons, polynuclear aromatichydrocarbons, aliphatic and aromatic organo-chlorinated products,pesticides, dioxins and dibenzofurans.
 11. The process according toclaim 1, which is carried out batchwise or in continuous fashion.